The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Various known MRI techniques have been found useful in aiding medical diagnosis. Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Nuclei in the subject have spins which in the presence of the main magnetic field produce a net magnetization. The nuclei of the spin system precess in the magnetic field at the Larmor frequency, i.e., the resonant frequency. Radio frequency (RF) magnetic fields at and/or near the resonant frequency are used to manipulate the net magnetization of the spin system.
Among other things, RF magnetic fields at the resonant frequency are used to, at least partially, tip the net magnetization from alignment with the main magnetic field into a plane transverse thereto. This is known as excitation, and the excited spins produce a magnetic field, at the resonant frequency, that is in turn observed by a receiver system. Shaped RF pulses applied in conjunction with gradient magnetic fields are used to manipulate magnetization in selected regions of the subject and produce a magnetic resonance (MR) signal. The resultant MR signal may be further manipulated through additional RF and/or gradient field manipulations to produce a series of echoes (i.e., an echo train) as the signal decays. The various echoes making up the MRI signal are typically encoded (i.e., phase encoded and frequency or read encoded) via magnetic gradients set up in the main magnetic field.
The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data line""s position in k-space (i.e., its relative k-space row) is typically determined by its gradient encoding. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space data.
As opposed to two dimensional (2D) imaging techniques which excite a 2D cross-sectional slice with each excitation, three dimensional (3D) or volume imaging refers to the practice of exciting a 3D volume within the subject. The resulting MR signal(s) are then sampled into a 3D k-space matrix with each slice therein typically having a distinct phase encoding position. In general, techniques for 3D or volume imaging are well known in the art. For example, 3D imaging techniques have been found particularly useful in studying blood vessels and blood or fluid flow via what is known as 3D magnetic resonance angiography (MRA) and/or magnitude contrast or time-of-flight (TOF) techniques.
When 3D imaging methods are employed to produce an MRA image, the size of the excited volume becomes a limiting factor. For example, to maximize the diagnostic value of an MRA image, it is advantageous to have a large volume thickness and in turn a large field of view along a general direction of blood flow. However, TOF and/or MRA images decrease in quality as the volume thickness increases due to the saturation of the spins as they flow through the excited volume. That is, due to the increased thickness of the excited volume, blood remains in the volume for a longer time and becomes saturated by the selective RF excitation pulse. As a result, fresh blood entering the volume appears much brighter in the reconstructed image than blood which has remained in the volume for a number of excitations.
Accordingly, one approach that has been developed involves acquiring a series or set of contiguous thin 3D slabs and stacking them together to form the entire volume of interest. However, this can lead to what is known as the slab boundary artifact (SBA) which gives a xe2x80x9cvenetian blindxe2x80x9d appearance to the resulting image. The SBA artifact is characterized by signal loss at the slab boundaries due to, e.g., imperfect slab excitation profiles, is flow dependent, and results in a signal intensity oscillation along the blood vessels. In turn, the artifact can result in, e.g., a false depiction of lumen diameter, over estimation of stenosis and/or atherosclerosis in clinical MRA images, etc.
One way to address the SBA problem is to overlap, to some degree, adjacent slabs. The acquisition technique is sometimes referred to as a multiple overlapping thin slab acquisition (MOTSA). An example of such a technique is found in U.S. Pat. No. 5,167,232 to Parker, et al., incorporated herein by reference in its entirety. The overlap method, however, has its own limitations. For example, it is accompanied by over sampling along the direction of the slab thickness or overlap, i.e., typically the z-axis. This over sampling results in inefficient data acquisition and can significantly increase the scan time relative to the amount of time it would have otherwise taken for the same overall volume of interest. Also, such oversampling only relieves the static SBA which is caused by, e.g., an imperfect RF pulse. It does not address dynamic SBA caused by flow conditions which are, at the very least, extremely difficult to predict.
In part to address the SBA and above problems, so called interleaved phase encoding acquisitions have been developed, such as the SLiding INterleaved ky (SLINKY) 3D acquisition technique and Shifted Interleaved Multi-Volume Acquisition (SIMVA or SIMUVA) for 3D fast spin echo (FSE). See for example, U.S. Pat. Nos. 6,037,771 and 6,043,654 to Liu, et al., both incorporated herein by reference in their entirety. These techniques have been found advantageous. However, the full extent of possible implementations for interleaved phase encoding acquisitions has not been completely appreciated in the prior art. That is to say, e.g., the artifact suppression factor, kas, (described later herein) has been fix at 100% in certain implementations. As a result, the flexibility of imaging parameter selection as well as scan time efficiency has been undesirably limited. In this regard, the present invention represents a supplementary extension of and improvement over the techniques disclosed in the aforementioned patents.
In short, the present invention contemplates an improved interleaved phase encoding acquisition technique which builds upon the prior art techniques to provide for controllable artifact suppression and flexible MRI parameter selection and allow for an adjustable trade-off between SBA suppression and scan time efficiency.
In accordance with one aspect of the present invention, a method of a conducting a magnetic resonance imaging experiment with an interleaved phase encoding acquisition is provided. The method includes setting an interleave factor which is an imaging parameter that represents the number of interleaves in the interleaved phase encoding acquisition, and determining an artifact suppression factor. The artifact suppression factor is an imaging parameter that represents an amount of SBA suppression achieved in the imaging experiment being conducted. The method further includes determining if the artifact suppression factor falls within a range. When the artifact suppression factor falls outside the range, notification is provided, and when the artifact suppression factor falls inside the range, a sequence of progression is determined for an excitation slab employed in the imaging experiment.
In accordance with another aspect of the present invention, a magnetic resonance scanner for conducting a magnetic resonance imaging experiment with an interleaved phase encoding acquisition includes setting means for setting an interleave factor which is an imaging parameter that represents the number of different interleaves in the interleaved phase encoding acquisition. Suppression determining means determine an artifact suppression factor which is an imaging parameter that represents an amount of SBA suppression achieved. Deciding means determine if the artifact suppression factor falls within a range. When the artifact suppression factor falls outside the range, notification means provide notification, and when the artifact suppression factor falls inside the range, sequence determining means determine a sequence of progression for an excitation slab generated by the scanner.
One advantage of the present invention is the adjustable suppression of slab boundary artifacts.
Another advantage of the present invention resides in the ability to have a flexible imaging parameter selection in connection with an interleaved phase encoding acquisition so as to achieve an adjustable compromise between SBA suppression and scan time efficiency which can be optimized as desired for a particular imaging experiment.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.